Advances in the field of fiber optic lasers have been rapid and such lasers have been integrated into the industrial and manufacturing sectors. The achieved power levels of fiber optic lasers have grown rapidly to output power levels exceeding 10 kW for 1 μm lasers. Power levels of 2 μm lasers area also growing with output power levels expected to exceed 10 kW within the near future.
Fiber optics presents a unique challenge in cascaded laser architectures due to the waveguide-nature of the fiber. Since the light is confined to a specific geometric shape and readily propagates long distances in both axial directions, efficient coupling between two fiber lasers is often simple to achieve. This is an advantage for efficient, low-loss laser operation but presents a problem in the case of laser back reflections. Cascaded fiber optic lasers, and particularly cascaded high power fiber optic lasers, have a problem with back reflections from a second laser into a first laser that drives the second laser. This destabilizes laser operation and results in degradation of many laser properties such as emission wavelength and output power in the first laser. With high power lasers this can be catastrophic and lead to the destruction of the first laser.
Isolation between cascaded lasers is necessary for uncoupled, stable operation of each laser in turn. Current attempts at optical isolation in fiber lasers are essentially miniature mimics of their solid state devices. The input and output fibers within these devices launch into or out of free space to interact with an optical material providing directional isolation. These devices typically result in insertion losses on the order of 1 dB, or about 20%; which is unacceptable in high power laser operation.
Such insertion losses can be minimized in high power laser applications using existing optical isolation techniques between cascaded fiber lasers. However, these existing techniques of optical isolation introduce complexity and cost to cascaded, high power, fiber laser systems. They require heavy magnets, polarizing elements, and robust, Faraday rotator materials or expensive, precision optical coatings that lead to large package sizes, often prohibitively high losses (as high as 5 dB) and high costs in the order of several thousands of dollars. Additionally these prior art techniques of isolation are limited in power handling capacity due to the very strong electric fields and high intensities associated with high power laser beams.
The short-falls of existing fiber-based isolator technology have driven significant research into the development of an all-fiber isolator. Current all fiber isolators are fiber analogs of a solid state laser. Chirally coupled cores serve as polarizers and a rare-earth doped fiber, in conjunction with a large magnet, perform the Faraday rotation. These devices exploit the same phenomena as the solid state analogs and carry the same inherent weaknesses as the existing technology in addition to incorporating the weaknesses of optical fibers such as high core intensities.
In-the-fiber isolators are actually free-space devices manufactured on millimeter-scale, are limited in power handling (typically <50 watt) and incur excess loss. Such high power, pure fiber isolators are still subjects of research and development and are not widely used in the high power laser industry.
Another major source of optical coupling loss is geometrical. As an example, two fibers coupled end-to-end may not be precisely aligned, with the result that the two cores overlap somewhat. Light exiting the source fiber at a portion of its core that is not aligned with the core of the receiving fiber will not (in general) be coupled into the second fiber. While some such light will be coupled into the second fiber, it is not likely to be efficiently coupled, nor will it generally travel in an appropriate mode in the second fiber.
Very briefly, what is needed in the art is efficient, relatively inexpensive, strong optical Isolation between two fiber optic lasers while maintaining highly-efficient laser operation.